1. Field of the Invention
The present invention relates to a hard disk drive adopting the staggered servo writing method, and particularly to a method for staggered servo writing without necessitating direct current (DC) erasing.
2. Description of the Related Art
In the hard disk drives widely used as auxiliary storing devices for computer systems, data are stored in the tracks arranged concentrically on each disk surface and cylindrically with the same track number on all the disks; the tracks are divided into sectors consisting of data sectors for storing information and servo sectors for storing servo information. In order to read and write on the specific sectors, the head must be moved onto the desired track, which is called track seeking, while the track following means to control the head to correctly follow the track. Accordingly, in order to move onto the specific track, the head must recognize its own position relative to the tracks, in other words, it should be furnished with the information as to the relative position of the head, i.e. the servo information has an inherent pattern. The servo information is permanently recorded in advance on the disk by a servo writer during the manufacture of the disk drive. The typical servo writing techniques are the dedicated and the embedded techniques.
The dedicated servo writing technique writes the servo information on the entire surface of the most stable disk selected from among a plurality of disk surfaces, and the embedded servo writing technique inserts a predetermined number of servo sectors alternately between data sectors on each surface of all of the disks so as to write the servo information of the same type and format on the servo sectors. Besides, the embedded servo writing technique is classified into the non-staggered and the staggered embedded servo writing techniques (hereinafter defined as the "staggered servo writing technique"), the latter of which is widely adopted because of enabling the reduction of the write cycle time for the servo writing.
The tracks corresponding to heads 0-3 are arranged vertically on all four disk surfaces to form a cylinder with the same track number from the top disk to the bottom disk, in which the servo sectors are shifted by a given time interval from each other, which can be implemented by switching all heads each at a given time interval during one rotation. However, the above staggered servo writing technique is attended with unavoidable difficulties when manufacturing hard disk drives practically. For example, firstly, defective servo information recorded by employing the staggered servo writing technique can be caused by a defect of the servo writer itself, a defective working process and an accidental external impact, which results in the defective recording. Secondly, although the quality itself of the servo information may be perfect, if a defective magnetic layer of the disk surface itself accidentally coincides with the servo sector, errors in the servo information are caused. Therefore, if such defects are caused, the defective servo sector must be shifted from the original position to rewrite the servo information on the new position. When rewriting the servo information for the HDD adopting the staggered servo writing technique, DC erasing must always precede the rewriting of the servo information to avoid the confusion of the servo control caused by the simultaneous presence of both the preceding and the rewritten servo information.
The earlier servo data erasing methods for the hard disk drives adopting the staggered servo writing technique are as follows:
The first is a method to erase all disk surfaces by means of servo writer before rewriting the servo information after mounting the hard disk assembly (HDA).
The second is a method to subject the disks to a separate erasing process by means of a simple erasing device manufactured with a similar construction to the servo writer because the erasing device doesn't require the precision of the servo writer.
The third is a method to demagnetize all disk surfaces without or with opening the covers of the HDD by means of a simple erasing jig employing a strong permanent magnet.
The fourth is a method to collectively demagnetize hard disks by means of a strong permanent magnet after dismantling hard disk assemblies and thereafter reassembling hard disks.
However, these earlier methods are attended with the following problems:
In the first method, the servo writer is not only an expensive equipment, but also has the problems of cycle time running counter to the trend of the cycle time reduction, and the load of the servo writer is increased, thereby reducing the lifetime of the servo writer and increasing the cycle time.
The second method requires an additional investment to manufacture an erasing equipment and maintain an onerous erasing process.
In the third method, although the disks are demagnetized without removing the heads by a strong permanent magnet force, the properties of the heads are changed instead, resulting in further worsening the problem, and it becomes more difficult to demagnetize the disks by means of the permanent magnet because the magnetic coercive force of the disks are continuously improved.
The fourth method provides onerous work for dismantling hard disk drives many of which are defect free and not necessary to be dismantled, thereby being attended with unnecessary overhead expenses.
The following patents each disclose features in common with the present invention but do not teach or suggest the staggered servo writing method of the present invention: U.S. Pat. No. 5,627,946 to Strang Jr., entitled Hard Disk Format Using Frames Of Sectors To Optimize Location Of Servo Bursts, U.S. Pat. No. 5,612,834 to Strang Jr., entitled Servo Burst Controller For A Magnetic Disk, U.S. Pat. No. 5,452,284 to Miyagawa et al, entitled Optical Recording/Reproducing Medium And Apparatus Using Land And Groove Encoded Information Signals And Identification Signals Including Position Information, U.S. Pat. No. 5,440,474 to Hetzler, entitled Magnetic Recording Disk With Equally Spaced Servo Sectors Extending Across Multiple Data Bands, U.S. Pat. No. 5,608,587 to Smith, entitled Method Using Magnetic Disk Servo Pattern With Buried Identification Patterns, U.S. Pat. No. 5,448,429 to Cribbs et al., entitled Self-Servowriting Disk Drive And Method, U.S. Pat. No. 5,381,281 to Shrinkle et al., entitled Disk Drive System Using Multiple Embedded Quadrature Servo Fields, U.S. Pat. No. 5,267,907 to Hedeen, entitled Control System For Variation Of The Sheave Ratio In A Continuously Variable Transmission, U.S. Pat. No. 4,980,783 to Meir et al., entitled Apparatus For Automatically Applying Servo Track Data To A Selected Servo Surface Of A Hard Disk Within A Hard Disk Assembly.